1. Field of the Invention
The present invention relates to an image reading incorporating a luminance control circuit for controlling the luminance or light intensity of a light source.
2. Description of the Related Art
A facsimile machine or a photocopying machine incorporates an image reading unit or apparatus for reading images of a document paper for example.
Typically, an image reading apparatus comprises a light source such as an LED array for irradiating a document paper with light, and an image sensor (photoelectric converter) for detecting the light reflected from the document paper to generate image signals. Normally, the light source is provided with a luminance control circuit for adjusting the luminance of the light source, thereby enabling the image sensor to suitably read the images of the document paper.
More specifically, as shown in FIG. 5 of the accompanying drawings, a prior art luminance control circuit for a light source of an image reading apparatus comprises an operation amplifier OP, a transistor TR.sub.5 and a resistor R.sub.5. The non-inverting input of the operation amplifier OP is connected to a current controlling voltage supplier (not shown). The base of the transistor TR.sub.5 is connected to the output of the operation amplifier OP, whereas the collector of the transistor TR.sub.5 is connected to an LED array 31 (a light source) which in turn is connected to a power supply source V.sub.DD. The emitter of the transistor TR.sub.5 is grounded through the resister R.sub.5. Further, an intermediate portion between the emitter of the transistor TR5 and the resistor R.sub.5 is connected to the inverting input of the operation amplifier OP for feedback. Thus, the current through the LED array 31 varies by changing the current controlling voltage applied to the non-inverting input of the operation amplifier OP.
However, with the prior art luminance control circuit shown in FIG. 4, since use is made of an operation amplifier OP which is a relatively costly element in addition to a bipolar transistor TR.sub.5, the production cost of the image reading apparatus inevitably becomes relatively high.